month-view: only update selection if there's a start date

We can potentially not have an end date set, but there's always
a start date. If there isn't, it means the user clicked from
outside of month view, or at an invalid cell, in which cases we
shouldn't display the quick add popover anyway.

src/views/gcal-month-view.c

@@ -232,7 +232,7 @@ emit_create_event (GcalMonthView *self)
 
   GCAL_ENTRY;
 
-  if (!self->end_mark_cell)
+  if (!self->start_mark_cell || !self->end_mark_cell)
     GCAL_RETURN (FALSE);
 
   should_clear_end = FALSE;
@@ -534,7 +534,7 @@ update_month_cells (GcalMonthView *self)
           selection_start = self->start_mark_cell;
           selection_end = self->end_mark_cell;
 
-          if (selection_start || selection_end)
+          if (selection_start)
             {
               if (!selection_end)
                 selection_end = selection_start;